Right-Hand Attack Dog

In contrast to the generally sedentary Right-Hand-Cat, this is what happens when the villains' pet plays an active role as a guard/weapon against the heroes. Expect Fluffy the Terrible in terms of naming and for the villain to be a confirmed Fluffy Tamer. Typically, villains will either show a Nightmare Fetishist streak and be oblivious to how savage their pets are, and will treat them kindly, or else, they will use the pets to (perhaps even literally) Kick the Dog, sometimes by feeding underlings and other poor unfortunates to them. Finally, as the title suggests, when the creature in question isn't a fantasy animal, it's very likely to be some kind of canine.

Although he is about as fearsome and "attack dog"-like as the kitten Nagi believes he is, Tama is a White Tiger. He has actually attacked Hayate on several occasions and shown to be quite capable of being a threat if he wants to be.

In the same theme, Shiranui is honestly a kitten, but shown to be quite evil at times.

Azraël, from The Smurfs, could be at first glance thought off as a mere Right-Hand-Cat to Gargamel. But since the Smurfs are so small, Azraël is a very physical threat and ends up chasing them quite often, putting him right into this trope.

In Oliver and Company, ruthless loan-shark Sikes has a pair of vicious Dobermanns (pictured above), who go about intimidating both the main characters and their impoverished "master" Fagin, and follow Sikes' will with gusto.

Commander Kruge in Star Trek III: The Search For Spock keeps a Klingon reptilian "pet" next to his command chair aboard the Bird of Prey. In contrast to its vicious appearance, it doesn't do much besides growl at people, and gets killed in the battle with the Enterprise to give Krudge a Pet the Dog moment.

Voldemort's gigantic pet snake, Nagini in the Harry Potter books is one of these. She also has a non-combat function as her venom helped to keep him alive prior to getting his full power back. She's pretty much the only other creature toward whom he shows any evidence of caring. Which probably has more to do with her being one of his Horcruxes than anything else.

The Big Bad of the Andrakis series has a pet tiger that sat by his throne. A luckless messenger comes to report the defeat of their armies by the reincarnated hero; he looks up and the last thing he sees are the tiger's jaws closing on his face.

In the first Animorphs Megamorphs book, Visser Three had a creature called the Valeek, that was like an alien life form hunting dog. It was trained to find morph energy and then return to feed on energy made by the Visser's ship. The team eventually destroyed it by forcing it into the water.

Mederan in the Tracker episode "Blind Faith". He was Zin's bodyguard trapped in a dog's body. He had a habit of transforming into a Hell Hound via his Vardian powers and tearing people apart for Zin until Cole took him out.

While he's not a villain, and therefore doesn't attack the heroes, Interceptor, Shadow's loyal dog in Final Fantasy VI, is made out to be something like this. In battle, he'll occasionally block physical attacks directed at his master and then counterattack For Massive Damage. Outside of the actual gameplay, Shadow warns that he'll actually tear out your throat if you get too close. Relm seems to be able to play with him just fine though, and if Shadow is KO'd in battle, Interceptor will actually protect her instead...

Comic Relief badguy Don Corneo from Final Fantasy VII has some sort of bizarre ape-like monster named Aps under his Trap Door. Later in a sidequest in Wutai, Corneo replaces with it a dragonish monster. Both can be quite difficult bosses.

In The Simpsons, Mr. Burns has a pack of hounds which he likes to release on those who disturb him at home. There is an episode where Santa's Little Helper becomes one of his guard dogs, and in the end, he protects Bart against the rest of the pack.

The Johnny Test TOS episode "Dragons of Ashida". The title character has large lizards similar to Komodo dragons that he considers his pets. He uses them for hunting human beings and as security for his island.

In The Man Called Flintstone, the Green Goose and his pet sabretooth tiger, Ferocious.

Wakfu: Season 1 Big Bad Nox has Igôle, a weird mix of wolf, hyena and lizard, that he sends chasing The Hero Yugo and his brother Adamaï, tasking it to bring them back "without damaging them too much". Note that Igôle was originally Nox's family pet 200 years ago, but exposure to the Eliacube and Xelor magic have turned it into a deadly powerful beast, and maybe as insane as its master. In the end it reverts to its past self and gives up the chase after meeting a little girl who reminds it of Nox's youngest daughter, and decides to stay with her.